1. Field of the Invention
The present invention relates to fabric architectures and soft body armor constructed therefrom.
2. Description of the Related Art
Protective body armors such as those providing protection against ballistic and stab type threats have long been an area of significant interest. One challenge for body armor manufacturers is to provide adequate protection from a particular threat or threats that the wearer may be subjected to in the field, while minimizing the weight, or areal density of the protective garment so as not to impede the dexterity of the wearer.
Characterization of the protective capabilities of any armor material against ballistic projectile threats, such as deformable bullets and non-deformable shrapnel fragments, requires some determination of the ballistic velocity limit with respect to the material's areal density and size, as well as the properties of the projectile (mass, hardness, shape, etc.). One common ballistic limit performance criteria is the ballistic V50, or the velocity at which 50% of the projectiles can be defeated by the armor. Specific testing and calculation protocols for determining V50 of body armors are outlined by the National Institute of Justice (NIJ) Standard 0101.06 Ballistic Resistance of Personal Body Armor, dated Jul. 2008. Beyond the ability of armor to stop the penetration of a projectile, the need to minimize blunt trauma associated with the ballistic impact for concealable body armors worn by police, security, and correctional officers, becomes an additional safety requirement set forth by NIJ Standard 0101.06. This standard outlines the testing protocol and performance requirements for an acceptable level of blunt trauma through measurement of the Back Face Deformation associated with ballistic impact of armors placed upon a clay witness simulation material. In NIJ Standard 0101.06, the acceptable amount of Back Face Deformation is defined as being no greater than 44 mm in a clay witness (Roma Plastilina clay, 5.5 in (140 mm) clay witness depth).
The NIJ Standard 0101.06 defines ballistic requirements specific to different types of projectiles and impact energy levels. Three common NIJ threat levels for soft body armor include Threat Level II, IIA, and IIIA. Threat level II relates to higher velocity 357 magnum, 10.2 g (158 gr) and 9 mm, 8.0 g (124 gr) bullets (impact velocities of less than about 1400 ft/s (427 m/s) and 1175 ft/s (358 m/s), respectively). Level IIA relates to lower velocity 40 S&W caliber full metal jacket bullets, with a nominal mass of 11.7 g (180 gr) and 9 mm 8.0 g (124 gr) bullets, (impact velocities of less than about 1025 ft/s (312 m/s) and 1090 ft/s (332 m/s), respectively). Threat level IIIA relates to 44 magnum, 15.6 g (240 gr) and sub machine gun 9 mm (124 gr) bullets having impact velocities of less than about 1400 ft/s).
In addition to the bullet type deformable threats described above, many types of body armors must also demonstrate the ability to stop non-deformable fragmentation type threats, such as those associated with the detonation of explosives.
The development of flexible body armor systems with multi-threat ballistic resistance to bullets and fragmentation threats as well as providing adequate blunt trauma protection against high energy bullets such as the 44 Magnum often require hybrid constructions of two or more high strength fiber ply structures with each type of ply structure being specifically suited for the defeat of a particular class of threat or impeding Back Face Deformation. This approach to soft body armor development can become an inefficient development strategy as body armor requirements drive toward increased protection against a diverse and growing variety of threats, while simultaneously trying to reduce the overall areal density of the body armor.
While the ballistic performance requirements set forth above can be achieved using any of several commercially available anti-ballistic materials, either alone or in combination, the challenge for soft body armor manufacture is the selection and arrangement of ballistic layers required to (1) prevent penetration with an acceptable safety margin, (2) minimize Back Face Deformation, (3) minimize the weight, bulk and stiffness of the armor to improve comfort and (4) reduce cost.
Commercially available anti-ballistic materials include a variety of woven fabrics, fabric reinforced composites, unidirectional fiber laminates and nonwovens. Of these various constructions, woven fabrics fabricated from high tenacity fiber yarns have the longest history of use in soft body armor fabrication. Weaving has long been a relatively inexpensive means of uniformly generating fabric ballistic resistant plies from high tenacity fiber yarns, relying on mechanical interlocking or “interlacing” of the yarns to hold the yarns in place instead of chemical locking by adhesive resins which can contribute additional weight and stiffness to a garment. Soft body armors fabricated from ballistic resistant fabrics are very often more conformable and flexible during use, providing greater comfort than hybrid armors containing stiff backface control layers such as unidirectional fiber laminates or resin impregnated fabrics. Additionally, it has been shown that ballistic resistant garments generated entirely of woven high tenacity fiber yarns maintain ballistic resistant properties after years of service and wear. Alternatives to an all woven ballistic resistant vest are in commerce. Such articles are prepared from combinations of high tenacity fibers, matrix resins and films, often making them more costly to produce. Additionally, by virtue of the component materials having temperature and strain dependent physical properties (eg. coefficient of thermal expansion, modulus, etc.) dissimilar to that of the ballistic fiber, these composite layers often have a useable life cycle dictated by the weakest of the materials selected.
Typical biaxial woven ballistic resistant fabrics (fabrics consisting of interwoven or interlaced yarns having two yarn orientations within the plane of the fabric) are generated on automated looms. These looming operations generate woven fabrics having interwoven fill fiber yarns oriented 90 degrees to those yarns in the warp, or machine direction. The fabric properties are largely governed by five basic variables: yarn mechanical properties, yarn denier, yarn count, weave pattern and fabric finish. Meeting the minimum ballistic performance requirements using only the above woven fabrics presents a challenge for ballistic armor manufacturers. While many low cover factor (loosely woven) ballistic resistant fiber yarn fabrics provide satisfactory V50 performance at the desired areal density (vests fabricated therefrom can be shown to repeatedly impede projectiles from penetrating the vest material at velocities safely above the threshold values outlined in NIJ Standard 0101.06), they do not provide adequate Back Face Deformation resistance. Conversely, the use of higher cover factor (more tightly woven) ballistic resistant fabrics at the same vest areal density while improving Back Face Deformation performance, often results in significant reduction in V50 performance, sometimes falling below the NIJ Standard 0101.06 velocities required for Back Face Deformation measurement. Currently no all p-aramid woven fabric vests are available commercially at an areal density of less than 4.93 kg/sq.m. (1 lb/sq.ft.) that can meet the NIJ Standard 0101.06 level IIIA backface requirement for a 44 magnum ballistic threat.
One common method for reducing the Back Face Deformation in soft body armors is through incorporating rigid plies of high tenacity fiber or fabric reinforced resin composite plies to impede deformation during impact. This includes bonding polymeric films or applying polymeric coatings to woven ballistic fabrics, or bonding two woven fabric layers to provide an anti-ballistic ply that can be added to ballistic body armor constructions to improve Back Face Deformation. Such an approach is described in PCT publication WO 00/08411, U.S. Pat. No. 5,677,029, and US patent application publication 2003/0109188. Resin or elastomer impregnated ballistic fiber fabric is another type of composite ply added to ballistic vest constructions to improve ballistic Back Face Deformation. While the addition of these layers has been shown to improve the Back Face Deformation performance of an armor material, they can often have a deleterious effect on V50 performance. In addition, the resin adds to the weight and stiffness of the ballistic vest assembly.
Unidirectional fiber laminates, comprised of a first plurality of parallel oriented high tenacity fibers in a polymeric matrix adhesively bound to a second plurality of parallel oriented high tenacity fibers in a polymeric matrix, where the fiber orientation of the second plurality is 90 degrees rotated relative to the orientation of the first plurality, have become popular anti-ballistic materials that can provide good backface trauma control while maintaining safe V50 performance. Methods of making these unidirectional fiber laminates are generally described in U.S. Pat. Nos. 4,916,000; 4,748,064; 4,737,401; 4,681,792; 4,650,710; 4,623,574; 4,563,392; 4,543,286; 4,501,854; 4,457,985, and 4,403,012. These unidirectional laminates are commercially available under the trade names Spectra Shield® Plus Flex, and Gold FIex™, from Honeywell International, Inc. and Dyneema®UD from DSM. While these unidirectional fiber laminates can be used alone to provide ballistic protection against some ballistic threats, it has been shown that further reductions in areal density and protection against a broad range of threats can be achieved when these materials are used in conjunction with woven ballistic fiber yarn fabrics, as illustrated in U.S. Pat. No. 6,119,575.
Performance improvements associated with using unidirectional fiber or fabric and resin composite layers in vests can be very dependent on their location within the multi-ply construction, as discussed in U.S. Pat. No. 6,119,575. In many documented instances, the placement of these stiffer composite layers behind traditional ballistic fabrics provides the optimum in Back Face Deformation and V50 performance. Due to this “sidedness” these hybrid ballistic vest constructions can be inadvertently worn inside-out, or inserted the wrong way into a tactical vest, providing less than optimal protection from projectile threats. Hence there is value in monolithic (comprised of all the same plies of anti-ballistic material) or front-back symmetric ballistic resistant armor constructions.
Triaxial fabrics, or woven fabrics comprised of three yarns are known. U.S. Pat. No. 1,368,215 to Stewart, U.S. Pat. Nos. 3,446,251 and 3,874,422 to Dow, and U.S. Pat. No. 4,438,173 to Trost all teach triaxial fabric structures. In U.S. Pat. No. 5,437,538 to Mitchell, the use of triaxial braided fabrics generated from Kevlar® fiber in blade containment projectile shield structures for gas turbine engines is disclosed. While no impact test method is provided in U.S. Pat. No. 5,437,538, Mitchell discloses that the ballistic resistance of a multi layered containment shield comprised of the triaxial braided fabric demonstrated containment performance comparable to a conventional woven fabric containment shield at a weight savings of 23%. Typical ballistic impact testing to determine the performance of turbine engine containment shields utilize relatively large projectile simulators at sub-sonic impact velocities representative of spall from fractured aircraft turbine blades, as described in the report “FAA Development of Reliable Modeling Methodologies for Fan Blade Out Containment Analysis” (Authors: Revilock and Pereira, NASA Glenn Research Center). It is well understood in the field of armor development that the impact physics associated with these large sized and low velocity (sub-sonic) projectiles is very different from that of significantly smaller and higher (supersonic) projectiles such as bullets and blast fragmentation from explosives.
Experimental investigations and computer simulations describing the resistance of triaxial fabric targets to bullet and fragmentation ballistic threats at super sonic velocities of importance to body armor design exist in the literature. In the work of Hearle et al. (Hearle, J. W. S., C. M. Leech, A. Adeyefa, C. R. Cork. 1981 “Ballistic Impact Resistance of Multi-layer Textile Fabrics”) the experimental results of the triaxial fabrics tested demonstrate inferior high velocity fragmentation ballistic resistance compared to biaxial fabrics. Computer simulations in the second part of the report by Hearle and coworkers further predict that the ballistic performance of triaxial fabrics should be inferior to the performance of typical biaxial woven fabrics for high velocity bullet and fragmentation threats. The published results from ballistic impact simulations performed on woven fabric composite constructions by Yen and Caiazzo (Yen, C. F., and A. A. Caiazzo. 2001 “3D Woven Composites for New and Innovative Impact and Penetration Resistant Systems” Technical Progress Report, Material Sciences Corporation, prepared for U.S. Army Research Office, Contract No. DAAD19-00-C-0107) also indicate that the impact resistance of triaxial braided fabrics are inferior to that of biaxial fabrics.
The need still exists for a lightweight body armor comprised of fabrics that can stop penetration and reduce the blunt trauma associated with high energy bullets, and, at the same time, provide improved protection against high velocity fragmentation threats. This is currently a challenge for body armor comprised entirely of traditional biaxial woven fabrics. The inventive triaxial braid fabric architectures described herein have demonstrated ballistic resistance improvement over conventional biaxial and other woven fabrics when tested against high velocity (super-sonic) high energy bullets and fragmentation projectiles, which is unanticipated by earlier impact investigations of triaxial woven fabrics.